1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a high contrast ratio.
2. Discussion of the Related Art
The cathode-ray tube (CRT) was developed and is mainly used for display systems. However, flat panel displays are beginning to be incorporated into display systems because of their small dimension, low weight and low power consumption. Presently, thin film transistor-liquid crystal displays (TFT-LCD) having a high resolution are being developed.
In general, Liquid Crystal Display (LCD) devices have various advantages in that, for example, they are relatively thin and require low power for operation, when compared to CRT display devices. Therefore, such LCD devices are good candidates to replace CRT display devices and have been a matter of great interest in a variety of technical fields.
Liquid crystal displays are classified into transmission types and reflection types depending on whether an internal or external light source is used. The transmission type has a liquid crystal display panel that does not itself emit light, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The liquid crystal panel controls the amount of the light, which is generated from the backlight and passes through the liquid crystal panel, in order to implement an image display. In other words, the light from the backlight selectively passes through the LCD panel and the LCD displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
To overcome the problems described above, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light and has a low power consumption.
The conventional transflective liquid crystal display device adopts a normally white mode in which the transflective device displays a white color when a signal is not applied. However, since the transflective liquid crystal display device is generally designed concentrating on the reflective mode, only about 50% of the light generated from the backlight device can pass through the liquid crystal display panel when the signal is not applied. Accordingly, the transflective LCD device often produces a gray color in operating.
To overcome the gray color problem, the transflective liquid crystal display device has different liquid crystal cell gaps between in the reflective portion and in the transmissive portion. FIG. 1 is a schematic cross-sectional view of a conventional transflective LCD device having a transmissive portion and a reflective portion.
In FIG. 1, the transflective LCD device divided into the transmissive portion A and the reflective portion B, and includes lower and upper substrates 10 and 60. A liquid crystal layer 100 having optical anisotropy is interposed between the lower and upper substrates 10 and 60.
The lower substrate 10 includes a first passivation layer 20 on its surface facing into the upper substrate 60. The first passivation layer 20 is made of an organic material and has a first transmitting hole 22 corresponding to the transmissive portion A. A transparent electrode 30 of transparent conductive material is disposed on the first passivation layer 20. A second passivation layer 40 and a reflective electrode 50 are sequentially formed on the transparent electrode 30. As shown in FIG. 1, the reflective electrode 50 corresponds to the reflective portion B and has a second transmitting hole 52 that exposes the second passivation layer 40 in the transmissive portion A. Although not shown in FIG. 1, a thin film transistor (TFT) is formed on the lower substrate 10 and electrically connected to both the transparent electrode 30 and the reflective electrode 50.
The upper substrate 60 includes a color filter layer 61 on its surface facing into the lower substrate 10. A common electrode 62 is formed on the surface of the color filter layer 61 facing toward the lower substrate 10.
On the exterior surfaces of the lower and upper substrates 10 and 60, lower and upper retardation films 71 and 72 are disposed, respectively. Since the lower and upper retardation films 71 and 72 have a phase difference xcex/4 (xcex=550 nm), they change the polarization state of the incident light. Namely, the lower and upper retardation films 71 and 72 convert the linearly polarized light into the right- or left-handed circularly polarized light, and they also convert the right- or left-handed circularly polarized light into the linearly polarized light of which polarization direction may be 45xc2x0 or 135xc2x0. A lower polarizer 81 is disposed on the rear surface of the lower retardation film 71, and an upper polarizer 82 is disposed on the front surface of the upper retardation film 72. Optical axis of the upper polarizer 82 is perpendicular to that of the lower polarizer 81. A backlight device 90 that emits an artificial light is adjacent to the lower polarizer 81. Light generated from the backlight device 90 is used as a light source in the transmissive mode of the LCD device.
The liquid crystals interposed between the lower and the upper substrates 10 and 60 have a positive dielectric anisotropy such that the liquid crystal molecules are aligned parallel with the applied electric field. An optical retardation (xcex94nxc2x7d) of the liquid crystal layer 100 depends on refractive-index anisotropy and thickness of the liquid crystal layer 100. Therefore, the liquid crystal layer 100 has different cell gaps between in the transmissive portion A and in the reflective portion B. The first transmitting hole 22 of the first passivation layer 20 allows the liquid crystal layer 100 of the transmissive portion A to be thicker than that of the reflective portion B, and makes the brightness uniform in all over the LCD device. Advisably, the thickness of the liquid crystal layer 100 in the transmissive portion A is twice as large as that in the reflective portion B.
The liquid crystal display shown in FIG. 1 includes the organic passivation layer that has the opening therein to make the different cell gaps. Thus, it is possible for the LCD device to obtain a uniform transmissivity whether it is operating in the transmissive mode or in the reflective mode. The polarization state of the light passing through the LCD panel shown in FIG. 1 is illustrated with reference to FIGS. 2 and 3.
From the point of the optical axis, the X-Y-Z coordinates are defined as illustrated in FIG. 1. The Z-axis is a progressing direction of the light, and the X-Y plane is parallel with the lower and upper substrates 10 and 60. From the observer""s viewpoint at the bottom of the liquid crystal display device, the optical axis of the upper polarizer 82 is 135 degrees with respect to the X-axis, and the optical axis of the lower polarizer 71 is 45 degrees with respect to the X-axis. Therefore, when the observer is at the top of the liquid crystal display device, the optical axis of the upper polarizer 82 is 45 degrees with respected to the X-axis. Furthermore, since the second polarizer 72 has a phase difference xcex/4 (xcex=550 nm) and an optical axis along the X-axial direction, incident light having 45 degrees with respect to the X-axis is converted into a left-handed circularly polarized light, a left-handed circularly polarized light is converted into a linearly polarized light of 135 degrees from the X-axis, incident light 135 degrees with respect to the X-axis is converted into a right-handed circularly polarized light, and a right-handed circularly polarized light is converted into a linearly polarized light of 45 degrees along the left-handed direction. However, since the lower retardation film 71 has a phase difference xcex/4 (xcex=550 nm) and an optical axis along the Y-axial direction, incident light having 45 degrees with respect to the X-axis is converted into a right-handed circularly polarized light, a right-handed circularly polarized light is converted into a linearly polarized light of 135 degrees from the X-axis, incident light 135 degrees with respect to the X-axis is converted into a left-handed circularly polarized light, and a left-handed circularly polarized light is converted into a linearly polarized light of 45 degrees along the right-handed direction. The liquid crystal layer 100 disposed in the reflective region B of FIG. 1 has a optical retardation of xcex/4 and makes the polarized light right-handed.
FIGS. 2A and 2B are views illustrating the state of ambient light passing through components of the transflective LCD device of FIG. 1 when it is operating in a reflective mode.
FIG. 2A shows the state of the ambient light in the reflective mode when a signal voltage is not applied, i.e., the TFT (not shown) is turned OFF. The ambient light illuminates the upper linear polarizer 82. Only the portion of the ambient light that is parallel with the optical axis of the upper polarizer 82 passes through the upper polarizer 82 as linearly polarized light (45xc2x0 from x-axis of reference frame). The linearly polarized light is changed into left-handed circularly polarized light by the upper retardation film 72. The left-handed circularly polarized light passes through the upper substrate 60, through the color filter layer 61 and through the common electrode 62 without any polarization change. The left-handed circularly polarized light then passes through the liquid crystal layer 100 that has optical retardation of xcex/4. The left-handed circularly polarized light is then converted into linearly polarized light of which polarization direction is 45xc2x0 as it passes through the liquid crystal layer 100. The linearly polarized light is then reflected by the reflective electrode 50. Due to the reflection, the linearly polarized light changes its polarization direction from 45xc2x0 to 135xc2x0. The reflected linearly polarized light is converted back into a left-handed circularly polarized light as it passes through the liquid crystal layer 100. The left-handed circularly polarized light is then converted into a linearly polarized light of which polarization direction is 135xc2x0 as it passes through the upper retardation film 72. The linearly polarized light is parallel to the optical axis of the upper polarizer 82, and thus passes through the upper linear polarizer 82. Thus, the LCD device produces light having a white color.
FIG. 2B shows the state of the ambient light in the reflective mode when a signal voltage is applied, i.e., the TFT (not shown) is turned ON. In the ON-state, the liquid crystal layer 100 does not affect polarization state of the incident light. Thus, incident light passes through the liquid crystal layer without any change of polarization state.
Accordingly, the ambient light that passes through the upper polarizer 82 as linearly polarized light is converted into left-handed circularly polarized light by the upper retardation film 72. The left-handed circularly polarized light passes through the upper substrate 60, through the color filter layer 61, through the common electrode 62, and through the liquid crystal layer 100. The left-handed circularly polarized light is then reflected by the reflective electrode 50, which causes the left-handed circularly polarized light to become converted into right-handed circularly polarized light with a phase shift of 180xc2x0 via a mirror effect. The right-handed circularly polarized light then passes through the liquid crystal layer 100, through the common electrode 62, through the color filter layer 61, and through the second substrate 60. The right-handed circularly polarized light is then converted into linearly polarized light of having a polarization direction of 45xc2x0 as it passes through the upper retardation film 72. The linearly polarized light is perpendicular to the optical axis of the upper polarizer 82, and as such does not pass through the upper linear polarizer 82. Thus, the LCD device results in a black color.
FIGS. 3A and 3B are views illustrating the state of light from a backlight device passing through components of the transflective LCD device of FIG. 1 when it is operating in a transmissive mode.
FIG. 3A shows the state of the light from the backlight device in the transmissive mode when a signal voltage is not applied, i.e., when the TFT (not shown) is turned OFF. At this time, the liquid crystal layer 100 disposed in the transmissive portion A of FIG. 1 has a optical retardation of xcex/2 because the liquid crystal layer in the transmissive portion A is twice as thick as that in the reflective portion B.
The light from the backlight device enters the lower polarizer 81. As mentioned before, transmissive axis of the lower polarizer 81 is arranged perpendicular to that of the upper polarizer 82. Only the portion of the light that is parallel with the transmissive axis of the lower polarizer 81 passes through the lower polarizer 81 as linearly polarized light of which polarization direction is 45xc2x0. The resultant linearly polarized light is converted into right-handed circularly polarized light as it passes through the lower retardation film 71. Then, the right-handed circularly polarized light passes through the lower substrate 10 and through the transparent electrode 30 without any phase shift. Next, the right-handed circularly polarized light is converted into left-handed circularly polarized light as it passes through the liquid crystal layer 100, this being due to a optical retardation xcex/2 of the liquid crystal layer 100. The left-handed circularly light then passes through the common electrode 62, through the color filter layer 61 and through the upper substrate 60, without any phase shift. As the left-handed circularly polarized light passes through the upper retardation film 72, the left-handed circularly polarized light is converted into linearly polarized light of which polarization direction is 135xc2x0. The linearly polarized light is polarized parallel with the optical axis of the upper polarizer 82, and thus passes through the upper linear polarizer 82. As a result, the LCD device produces a gray color.
FIG. 3B shows the polarization state of the light from the backlight device in the transmissive mode when a signal voltage is applied, i.e., the TFT (not shown) is turned ON. The liquid crystal does not affect the incident light, and thus the incident light passes through the liquid crystal layer without any change of polarization state. The light from the backlight device enters the lower polarizer 81. Only the linearly polarized light of the light of which polarization direction is 45xc2x0 can pass through the lower polarizer 81. The resultant linearly polarized light is converted into right-handed circularly polarized light as it passes through the lower retardation film 71. Then, the right-handed circularly polarized light passes through the lower substrate 10, through the transparent electrode 30, and through the transmitting holes 22 and 52 without any polarization change. When the right-handed circularly polarized light passes through the liquid crystal layer 100, it is not converted and polarized any more because the liquid crystal layer 100 ideally does not have the optical retardation in the ON-state. The right-handed circularly polarized light then passes through the common electrode 62, through color filter layer 61, and through the upper substrate 60. As the right-handed circularly polarized light passes through the upper retardation film 72, it is converted into linearly polarized light of which polarization direction is 45xc2x0. This linearly polarized light is polarized perpendicular to the optical axis of the upper polarizer 82, and therefore, does not pass through the upper linear polarizer 82. Thus, the LCD device produces a black color.
As described before, since the conventional LCD device has different cell gaps between in the transmissive portion and in the reflective portion, the uniform brightness can be obtained whether it is operating in the transmissive mode or in the reflective mode. Furthermore, because it ideally displays the black colors when the voltage is applied, the contrast ratio increases. Accordingly, the LCD device has an improved image quality.
However, as shown in FIG. 1, the first transmitting hole 22 of the first passivation layer 20 causes the liquid crystal layer 100 to have different cell gaps, and moreover, the transmitting hole 22 has slanted portions therearound. Therefore, the cell gaps have altered continuously all over the slanted portions. Further, the liquid crystal molecules above the slanted portions are not aligned properly when the voltage is not applied. These cause the change of the optical retardation of the liquid crystal layer in the slanted portion, thereby promoting the light leakage in those slant portions. When the voltage is applied, the electric field is distorted in the slanted portions, thus the liquid crystal molecules are not aligned appropriately. The optical retardation of the liquid crystal layer also changes in the slanted portion when the voltage is applied, and the light leakage additionally occurs.
Accordingly, the present invention is directed to a transflective liquid crystal display and a method of fabricating the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a transflective LCD display and a method of fabricating the same preventing a light leakage.
Another advantage of the present invention is to provide a transflective LCD display and a method of fabricating the same achieving a high contrast ratio.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an array substrate for use in a transflective liquid crystal display includes a substrate including a transmissive portion, a reflective portion and a border region in a pixel region, wherein the border region is between the transmissive portion and the reflective portion; at least a gate line, a gate electrode and a first light-shielding patter on the substrate, wherein the first light-shielding pattern has a first opening corresponding in position to the transmissive portion and is disposed in the border region; a gate insulation layer on the substrate covering the gate line, the gate electrode and the first light-shielding pattern; an active layer on the gate insulation layer over the gate electrode; a second light-shielding pattern on the gate insulation layer, wherein the second light-shielding pattern has a second opening corresponding in position to the transmissive portion and is disposed in the border region; first and second ohmic contact layers on the active layer; at least a data line, a source electrode and a drain electrode, wherein the data line defines the pixel region with the gate line, and wherein the source electrode is disposed on the first ohmic contact layer and the drain electrode is disposed on the second ohmic contact layer; an inorganic insulation layer on the gate insulation layer, the inorganic insulation layer covering the data line, the source and drain electrodes and the second light-shielding pattern; and an organic passivation layer on the inorganic insulation layer; wherein the inorganic insulation layer and the organic passivation layer have a drain contact hole that exposes a portion of the drain electrode; and wherein the inorganic insulation layer and the organic passivation layer have a first transmitting hole that corresponds in position to the transmissive potion.
The above-mentioned array substrate may further include a first inorganic passivation layer on the organic passivation layer; a reflective plate on the first inorganic passivation layer; a second inorganic passivation layer on the first inorganic passivation layer to cover the reflective plate; and a transparent electrode on the second inorganic passivation layer. The reflective plate has a second transmitting hole that corresponds in position to both the transmissive portion and the border region. Also, the reflective plate is disposed in the reflective portion. The first and second inorganic passivation layers have an additional drain contact hole that exposes a portion of the drain electrode. The transparent electrode contacts the drain electrode through the additional drain contact hole. The first and second inorganic passivation layers are formed of an inorganic material selected from a group consisting of silicon nitride and silicon oxide. The reflective plate is made of a metallic material selected from a group consisting of aluminum and aluminum alloy. the transparent electrode is formed of a transparent conductive material selected from a group consisting of indium tin oxide and indium zinc oxide.
Beneficially, the organic passivation layer is formed of an organic material selected from a group consisting of benzocyclobutene and acryl-based resin. The first and second light-shielding patterns extend to the reflective portion. The first transmitting hole exposes the substrate by removing a portion of the gate electrode in the first opening.
The above-mentioned array substrate may further include a transparent electrode on the organic passivation layer, the transparent electrode contacting the drain electrode through the drain contact hole; an inorganic passivation layer on the organic passivation layer to cover the transparent electrode, the inorganic passivation layer having a contact hole that exposes a portion of the transparent electrode over the drain contact hole; and a reflective electrode on the inorganic passivation layer, the reflective electrode contacting the transparent electrode through the contact hole, the reflective electrode having a second transmitting hole that corresponds to both the transmissive portion and the border region. The reflective electrode is disposed in the reflective portion.
The above-mentioned array substrate may further include a reflective plate between the inorganic insulation layer and the organic passivation layer, the reflective plate having a second transmitting hole corresponds to both the transmissive portion and the border region; and a transparent electrode on the organic passivation layer, the transparent electrode contacting the drain electrode through the drain contact hole.
In another aspect of the present invention, a method of forming an array substrate for use in a transflective liquid crystal display (LCD) device includes the steps of providing a substrate that includes a transmissive portion, a reflective portion and a border region in a pixel region, wherein the border region is between the transmissive portion and the reflective portion; simultaneously forming at least a gate line, a gate electrode and a first light-shielding pattern on the substrate, wherein the first light-shielding pattern has a first opening corresponding in position to the transmissive portion and is disposed in the border region; forming a gate insulation layer on the substrate to cover the gate line, the gate electrode and the first light-shielding pattern; simultaneously forming an active layer and a second light-shielding pattern, wherein the active layer is disposed on the gate insulation layer over the gate electrode and the second light-shielding pattern is disposed on the gate insulation layer, and wherein the second light-shielding pattern has a second opening corresponding in position to the transmissive portion and is disposed in the border region; forming first and second ohmic contact layers on the active layer; simultaneously forming at least a data line, a source electrode and a drain electrode, wherein the data line defines the pixel region with the gate line, and wherein the source electrode is disposed on the first ohmic contact layer and the drain electrode is disposed on the second ohmic contact layer; forming an inorganic insulation layer on the gate insulation layer to cover the data line, the source and drain electrodes and the second light-shielding pattern; and forming an organic passivation layer on the inorganic insulation layer; and patterning both the inorganic insulation layer and the organic passivation layer to from a drain contact hole and a first transmitting hole, wherein the drain contact hole exposes a portion of the drain electrode, and wherein the transmitting hole corresponds in position to the transmissive potion.
The above-mentioned method further includes the steps of forming a first inorganic passivation layer on the organic passivation layer; forming a reflective plate on the first inorganic passivation layer; forming a second inorganic passivation layer on the first inorganic passivation layer to cover the reflective plate; and forming a transparent electrode on the second inorganic passivation layer. The reflective plate has a second transmitting hole that corresponds in position to both the transmissive portion and the border region. The reflective plate is disposed in the reflective portion. The method may further include the step of patterning both the first and second inorganic passivation layers to form an additional drain contact hole that exposes a portion of the drain electrode. The transparent electrode contacts the drain electrode through the additional drain contact hole. The first and second inorganic passivation layers are formed of an inorganic material selected from a group consisting of silicon nitride and silicon oxide. Beneficially, the reflective plate is made of a metallic material selected from a group consisting of aluminum and aluminum alloy, and the transparent electrode is formed of a transparent conductive material selected from a group consisting of indium tin oxide and indium zinc oxide.
Beneficially, the organic passivation layer is formed of an organic material selected from a group consisting of benzocyclobutene and acryl-based resin. The first and second light-shielding patterns can extend to the reflective portion. The above method further includes a step of removing a portion of the gate electrode located in the first opening to expose the substrate.
The above-mentioned method may further include the steps of forming a transparent electrode on the organic passivation layer, the transparent electrode contacting the drain electrode through the drain contact hole; forming an inorganic passivation layer on the organic passivation layer to cover the transparent electrode, the inorganic passivation layer having a contact hole that exposes a portion of the transparent electrode over the drain contact hole; and forming a reflective electrode on the inorganic passivation layer, the reflective electrode contacting the transparent electrode through the contact hole, the reflective electrode having a second transmitting hole that corresponds to both the transmissive portion and the border region. The reflective electrode is disposed in the reflective portion.
The above mentioned method may further include the steps of forming a reflective plate between the inorganic insulation layer and the organic passivation layer, the reflective plate having a second transmitting hole corresponding to both the transmissive portion and the border region; and forming a transparent electrode on the organic passivation layer, the transparent electrode contacting the drain electrode through the drain contact hole.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.